Add support for CUDA __launch_bounds__ attribute to CodeGen.

Sema does have a CUDALaunchBoundsAttr, but CodeGen was doing nothing with it.
This change translates CUDALaunchBoundsAttr to maxntidx and minctasm
metadata, which NVPTX then translates to the correct PTX directives.

Patch by Manjunath Kudlur.